1. Field of the Invention
The present invention relates to an apparatus and method for controlling a position of a control object in a numerical control (NC) machine tool or other machine.
2. Description of the Related Art
Positional control of a control object such as a work table and a work cutting tool in an NC machine tool is generally achieved by controlling rotation of a servomotor connected to the control object via a transmission mechanism constructed for example by a rack and pinion or a ball screw and nut.
As methods for positional control of a control object, a semi-closed-loop control system, a full-closed-loop control system, and a hybrid control system are known.
In the semi-closed-loop control system, the servo control system is constructed to detect the rotational position of the servomotor by a rotary encoder or other detector, convert the detected rotational position of the servomotor to the position of the control object, then feed back the converted rotational position to a variable position reference for controlling the rotation of the servomotor.
In the full-closed-loop control system, the servo control system is constructed to directly attach for example a linear scale to the table or other control object and feed back the position of the control object detected by the linear scale to the variable position reference for controlling the rotation of the servomotor.
In the hybrid control system, the servo control system is constructed to detect both the rotational position of the servomotor and the position of the control object, feed back the rotational position of the servomotor converted to the position of the control object to the variable position reference, and filter the difference between the position of the control object and the converted rotational position of the servomotor by a first order lag filter, then feed it back to the variable position reference to control the rotation of the servomotor.
When there is backlash, friction, or other a non-linear characteristics in the transmission mechanism arranged between the servomotor and the control object, even if the rotational position of the servomotor is accurately controlled, when the feed direction of the control object reverses, the servomotor rotates, but the control object remains stopped, i.e., so-called “lost motion” occurs, and the control object does not immediately track the variable position reference.
As a method for compensating for this tracking error, so-called “backlash elimination” for detecting reversal of the feed direction in the variable position reference, then correcting the control reference to the servomotor to quickly eliminate the lost motion and thereby suppress tracking error of the control object from the variable position reference is known.
In the above-mentioned semi-closed-loop control system, the position of the control object is indirectly obtained from the rotational position of the servomotor and is not directly managed, so no offset occurs in the control object even with backlash elimination.
In the above-mentioned full-closed-loop control system, however, if such backlash elimination is performed while the control object is moving, that is, right in the middle of the change of the feedback value from the linear scale, the positional information of the control object is corrected and offset in the position of the control object.
On the other hand, in the hybrid control system, since the feedback value of the servo motor is used in addition to the feedback value from the linear scale, correction of the feedback value of the servomotor can prevent occurrence of offset in the positioning of the control object.
However, in the hybrid control system, since the detected position of the linear scale is controlled to match with the target position, if the feedback value of the position of the control object from the linear scale changes even slightly in the reverse direction from the direction where lost motion occurs right before reversal of the direction of movement of the control object, irrespective of the fact that the variable position reference is unchanged, the servomotor moves the control object past the lost motion region and stops it when the feedback value of the position of the control object and the variable position reference match, thereby canceling the lost motion.
If reversal of the feed direction in the variable position reference is detected from this state, however, backlash elimination kicks in and the control object ends up deviating from the target position for an instant. If such a phenomenon occurs, when controlling the control object to follow an arc by employing orthogonal double-axis control, the trajectory of the control object ends up cutting in from the arc at so-called quadrant switching points.